In the field of optical communications, digital coherent communication systems, which are based on a combination of a synchronous detection method and digital signal processing, are attractive for rapidly improving the frequency usage efficiency. It is known that, when compared with the system constructed with direct signal detections, the digital coherent communication systems can better improve the receiver sensitivity, by receiving signals as digital signals, and compensate the distortion of transmission signals owing to chromatic dispersion caused by optical fiber transmission and polarization mode dispersion (PMD). Thus, the digital coherent communication systems are considered as the next generation optical communication technology and being developed for introduction.
Today, the long-distance transmission of optical communications is performed by multiple connections of optical fiber amplifiers. It is known, in such communication systems, that the data transmission characteristics degrade with the polarization dependence caused by the polarization hole burning (PHB) effect of optical fiber amplifiers. The polarization hole burning is a phenomenon that amplification factors in the direction of signal light propagation and the orthogonal direction to the signal light propagation become different from each other when signal light is incident to an optical fiber amplifier. This degrades the signal-to-noise ratio (S/N ratio) because spontaneous light (amplified spontaneous emission: ASE) in the orthogonal direction is amplified to a higher power level than the other directions for natural light.
A polarization wave scrambler that actively changes the polarization of signal light on the transmission side is proposed as a technique that reduces the polarization hole burning effect, which is, for example, described in patent document 1.
FIG. 27 is a perspective diagram that illustrates an example of the construction of a conventional polarization scrambler. In FIG. 27, a crystal substrate 1 is made from lithium niobate providing an electric-optical effect that changes its refractive index according to the applied voltage. An optical waveguide 2 and electrodes 3 and 4 are formed on the crystal substrate 1. The state of polarization of incident light is changed into linear polarization by use of a polarizer 4, and introduced to the optical waveguide 2 at a 45-degree inclined angle to the cross-section axis of the optical wave guide 2. S1 indicates the state of polarization of the incident light.
As refractive indexes of the horizontal and vertical components of the optical waveguide 2 change according to the applied voltage to the electrodes 3, the propagation speed of the horizontal and vertical components of the linear polarization light in the optical waveguide 2 change according to the applied voltage to the electrodes 3, so that a phase difference between the horizontal and vertical components of signal light changes according to the applied voltage. Thereby, when the applied voltage is varied with time, the state of polarization of the signal light can be randomized. Further, the signal light is transmitted through a polarizer 5. In the figure, S2 indicates the state of polarization of the output light.
FIG. 28 is a block diagram that illustrates an example of the construction of a conventional transmitter/receiver apparatus for conventional optical communications. FIG. 28 shows a semiconductor laser 300, a transmitter 301, an optical fiber 305, a receiver 306, and a local oscillation laser 309. The transmitter 301 includes a first modulator 302, a second modulator 303, a polarization beam combiner 304, and a polarization scrambler 326. Accordingly, the transmitter 301 includes the polarization scrambler 326.
The receiver 306 includes a first polarization beam splitter 307, a second polarization beam splitter 308, a first 90 degree hybrid 310, a second 90 degree hybrid 311, a first light receiving element 312, a second light receiving element 313, a third light receiving element 314, a fourth light receiving element 315, a first AD converter 316, a second AD converter 317, a third AD converter 318, a fourth AD converter 319, a first complex adding circuit 320, a second complex adding circuit 321, a combining circuit 322, a synchronous circuit 323, an equalizer 324, and a demodulor 325.
In the transmitter 301, a signal is modulated through the first modulator 302 and second modulator 303, polarization multiplexing is performed for the signal with the polarization beam combiner 304, and the signal is input to the polarization scrambler 326. After the state of polarization of signal light is randomized by the polarization scrambler 326, the signal light is input to the optical fiber 305 for transmission. In the receiver 306, the signal light is split into I-signal and Q-signal that are two orthogonal polarized waves through the first and second polarization beam splitters 307 and 308 and the first and second 90 degree hybrids 310 and 311, and then after the I-signal and Q-signal are converted to digital signals through the first, second, third and fourth AD converters 316, 317, 318 and 319, the digital signals are converted into complex signals, I+jQ, by use of the complex adding circuits 320 and 321 for each of polarized waves. The obtained complex signals are combined for each of polarized waves with the combining circuit 322 as polarization diversity. Further, the mixed signal is demodulated through the synchronous circuit 323, the equalizer 324, and the demodulator 325, and then the demodulated signals are output.
Next, another example of the construction of a conventional transmitter/receiver apparatus for optical communications will be described below.
The digital coherent method described in non-patent documents 1, 2 and 3 compensates a quasi-static wave dispersion with a fixed digital filter (e.g. for 28 Gbaud signals, the dispersion is 2000 ps/nm with 2048 taps), and the digital coherent method compensates polarization mode dispersion with fluctuations by an adaptive filter with a small number of taps (e.g. 10-12 taps for polarization mode dispersion of 50 ps) based on a blind algorithm.
In a coherent receiver/transmitter system using digital signal processing, wave distortions added in the transmission line are compensated by the digital signal processing of the receiving end. The amount of wave dispersion added through a transmission line greatly varies depending on the conditions of the transmission line. For example, the cumulative amount of wave dispersion increases in proportion to the length of a transmission line fiber in which the signal light has propagated, such that the amount of dispersion varies with the transmission distance. Further, even if the length of the transmission line fiber is known, since there are different types of transmission line fibers that include a single mode fiber, a dispersion shift fiber, a non-zero dispersion shift fiber or the like, dispersion per a unit length is different.
In some cases, optical dispersion compensators are used in the repeaters in the transmission systems, and the amount of residual dispersion is varied by the amount of compensation of the dispersion. In submarine system, dispersion compensation fibers can be used as transmission lines. In addition, since chromatic dispersion coefficients are varied with the carrier wavelength of signal light, the amount of cumulative dispersion depends on the wavelength of signal light.
According to the reasons above, with respect to the amount of dispersion compensation in the chromatic dispersion compensation circuit at a receiving end, the coefficient of dispersion compensation filters should be adjusted in response to the cumulative amount of chromatic dispersion which the signals have received. For this, a mechanism is required to estimate the cumulative amount of chromatic dispersion which the signals have received. For example, there is a method that estimates the amount of chromatic dispersion by use of a known signal at the receiving end in which the known signal is incorporated in the transmission signal light.
Further, the state of dispersion of the transmission line is random, and since the state varies with time, it is required to use an adaptive equalizer filter for separating polarized multiple signals, in which the coefficient of the filter needs to be properly adjusted. In addition, the coefficient of the adaptive equalizer filter needs adaptive control because the wave distortion due to the polarization mode dispersion varies with time. There is a method to control the coefficient of the adaptive equalizer filter, in which the pattern of the known signal is periodically inserted in the transmission data.
In general, the degradation of wave forms caused by an optical device impairment in a transmission line depends on the polarization of waves. For example, when there are polarization dependent loss, polarization mode dispersion, and polarization dependent gain, the degree of wave form degradation is changed. In some cases, there is a phenomenon that the amount of wave degradation due to non-linear distortion of an optical fiber waveguide changes depending upon the state of polarization of a neighboring channel. In other words, the wave form degradation depends on the relation between the polarization state of signal light and the principle state of polarization (PSP) of a medium having the polarization dependence. For example, there is a phenomenon that the wave degradation of polarization signals in one side becomes greater than that in the other side.
When a known pattern is inserted at a transmitting end and the coefficients of the dispersion compensation filter and the adaptive equalizer filter are controlled based on estimations by use of a known pattern at a receiving end, both X-polarization and Y-polarization become certain known patterns. Thereby, the optical electric field after combining an X-polarized wave and a Y-polarized wave takes a certain state of polarization. In this case, if the state of polarization after combination of the polarized waves corresponds to the polarization state of a large wave distortion at a transmission line, the wave distortion at part of the known pattern becomes remarkably large, so that a wrong estimation result is output for the channel estimation. Therefore, if the coefficients of the dispersion compensation filter and the adaptive equalizer filter are controlled based on the wrong estimation result, a wrong signal processing operation is performed in the compensation process of wave distortion, and the performance of wave distortion compensation is remarkably degraded.
In order to solve the problems described above, there is a technique that varies the polarization state of signal light with time at a transmission side by use of an optical polarization scrambler. The technique used a polarization scrambler having optical connectors as input/output. As the polarization scrambler affects variations of polarization for the whole data, the cycle of the polarization scrambler needs to be increased when it is used.
FIG. 29 is a block diagram that illustrates the construction of a transmitter/receiver apparatus for conventional optical communications. FIG. 29 shows a semiconductor laser 300, a transmitter 301, an optical fiber 305, a receiver 306, and a local oscillator 309. The transmitter 301 consists of a first modulator 302, a second modulator 303, and a polarization beam combiner 304.
The receiver 306 includes a first polarization beam splitter 307, a second polarization beam splitter 308, a first 90 degree hybrid 310, a second 90 degree hybrid 311, a first light receiving element 312, a second light receiving element 313, a third light receiving element 314, a fourth light receiving element 315, a first AD converter 316, a second AD converter 317, a third AD converter 318, a fourth AD converter 319, a first complex addition circuit 320, a second complex addition circuit 321, a combining circuit 322, a synchronous circuit 323, an equalizer 324, and a demodulator 325.
In the transmitter 301, a signal is modulated through the first modulator 302 and second modulator 303. After polarization multiplexing is performed for the signal with the polarization beam combiner 304, the signal is input to the optical fiber 305 for transmission. In the receiver 306, the signal light is split into I-signal and Q-signal that are two orthogonal polarized waves through the first and second polarization beam splitters 307 and 308 and the first and second 90 degree hybrids 310 and 311, and then after the I-signal and Q-signal are converted to digital signals through the first, second, third and fourth AD converters 316, 317, 318 and 319, the digital signals are converted into complex signals, I+jQ, by use of the complex adding circuits 320 and 321 for each of polarized waves. The obtained complex signals are mixed for each of polarized waves with the combining circuit 322 as polarization diversity. Further, the mixed signal is demodulated through the synchronous circuit 323, the equalizer 324, and the demodulator 325, and then the demodulated signals are output.